The present invention relates to a lamp base for use with a fluorescent lamp and a fluorescent lamp which comprises such lamp base, and in particular to a lamp base which reduces soluble mercury in a lamp to an acceptable level.
Fluorescent lamps contain elemental mercury. During lamp operation, chemical reactions take place that convert some of the elemental mercury to salts or compounds such as mercuric oxide (HgO) that are water soluble. There is a growing concern that a waste stream resulting from the disposal of fluorescent lamps may leach excessive amounts of this soluble form of mercury (Hg). The method of measuring the amount of soluble mercury which may leach from the waste stream resulting from the disposal of fluorescent lamps is described in the Toxicity Characteristic Leaching Procedure (TCLP) prescribed on pages 26987-26998 of volume 55, number 126 of the Jun. 29, 1990 issue of the Federal Register. At the present time, the United States Environmental Protection Agency (EPA) defines a maximum concentration level for mercury at 0.2 milligram leachable mercury per liter leachate fluid when the TCLP is applied. According to the present standards, a fluorescent lamp is considered nonhazardous when less than 0.2 milligram per liter of leachable mercury results using the TCLP. In addition to leaching Hg into the waste streams, disposal operators charge a fee for disposal of lamps that are not within the EPA""s limits. Therefore, customers must pay to dispose of these lamps. Customers of fluorescent lamps generally do not desire to have to contend with the EPA and disposal concern regarding mercury levels, and therefore some customers specify only those lamps which pass the TCLP standard.
Heretofore, efforts have been made to reduce the leaching of soluble mercury during the TCLP as well as in landfills. Various methods have been proposed which attempt to treat or process burned-out discharge lamps or scrap lamp exhaust tubing containing mercury in order to reclaim the mercury and thereby reduce the amount of mercury-contaminated scrap. These methods are summarized as background in U.S. Pat. Nos. 5,229,686 and 5,229,687 which were granted to Fowler et al. on Jul. 20, 1993. These two patents are commonly owned with the instant application (GTE Products Corporation having changed its name to Osram Sylvania Inc.), and are incorporated herein by reference. These patents describe methods by which to render a mercury vapor lamp nonleaching upon disposal without the use of expensive treatment processes to reclaim the mercury. The method of the U.S. Pat. No. 5,229,686 patent employs a chemical agent, enclosed within the lamp, suitable for chemically combining a substantial portion of the soluble mercury as a sparingly soluble salt when the lamp is pulverized during disposal. The method of the U.S. Pat. No. 5,229,687 patent employs a chemical agent, enclosed within the lamp, suitable for electro-chemically reducing a substantial portion of the soluble mercury to elemental mercury, when the lamp is pulverized during disposal. Preferably, this chemical agent is an element which has an electrode potential for oxidation reactions higher than mercury but which is not sufficiently active to displace hydrogen from acidic aqueous solutions. In one embodiment, the chemical agent is sealed within an enclosure (e.g., glass) which is rupturable upon pulverization of the lamp. In another embodiment, the chemical agent is mixed with the basing cement used to secure the lamp bases to the glass envelope. The chemical agent acts to reduce soluble mercury produced during lamp operation to elemental mercury that is not leachable as measured by the TCLP.
The chemical agent used in the U.S. Pat. No. 5,279,687 patent may be used in various forms, e.g., as a powder, dust, wire mesh, or metallic foil. The amount or size of the chemical agent is directly related to its surface area and surface condition, finely divided metallic powders being preferred over a solid mass because of their relatively large effective surface areas. Because of their availability and inexpensive cost, iron and copper, in the form of a powder or dust, are preferred. The amount of chemical agent present should be sufficient to electrochemically reduce the amount of soluble mercury within the lamp, which is leached at the time of disposal, to less than 0.2 milligram per liter of an aqueous acid solution such as a sodium acetate buffer solution as prescribed in the TCLP.
Although the methods described in the U.S. Pat. Nos. 5,229,686 and 5,229,687 patents provide generally satisfactory performance, they have been found to have certain disadvantages. For example, in considering the U.S. Pat. No. 5,229,686 patent, in some fluorescent lamp applications the quantity of chemical agent required to chemically combine nearly all of the mercury within the lamp may be so large as to be inconvenient or impossible to contain within a standard lamp envelope. In considering the U.S. Pat. No. 5,229,687 patent, in some fluorescent lamp applications the metallic copper or iron reduces the amount of leachable mercury via a surface reduction-oxidation reaction between adsorbed mercury ions and zero-valent metal atoms. In order for this reaction to occur, the dissolved ionic mercury must first find its way to and become adsorbed upon the metal surface. Thus, the effectiveness of a metallic element as a means of reducing leachable mercury is limited by the rates at which mercury ions diffuse to the metal surface and become adsorbed thereon. Increasing the surface area of the metallic elements described in the U.S. Pat. No. 5,229,687 patent to improve the chance of contact between dissolved mercury ions and a metal surface followed by the adsorption of the mercury upon that surface is not a feasible alternative. For example, it may be difficult or impossible to incorporate a sufficiently large quantity of a high surface area agent such as finely divided metal within a fluorescent lamp, the more so the smaller or more compact the lamp. In a small lamp, the only convenient way to introduce the metal may be as a component of the basing cement. However, the electrical conductivity of the metal may prevent its incorporation into the basing cement since the cement may easily come into contact with internal electrical leads. Although electrically insulating materials might be added to the basing cement in addition to or in place of the normal CaCO3 cement filler without risk of creating electrical short circuits within the lamp, such an addition adds to the cost.
In U.S. Pat. No. 5,736,813, which was granted to Foust et al. on Apr. 7, 1998, it is stated that the formation of leachable mercury during TCLP testing or during disposal of mercury vapor discharge lamps may be substantially prevented by incorporating a pH control agent in the lamp structure or in the test solution to provide a pH of about 5.5 to 6.5. A low pressure mercury discharge lamp is described which includes about 5-15 grams of a pH control agent (generally a water-soluble base) which, it is suggested, is sufficient to substantially prevent formation of ferric and cupric compounds which oxidize elemental mercury to a soluble form. The primary disadvantage of this method of reducing mercury leaching is that it may be difficult or, depending upon the lamp type, practically impossible to package the relatively large amounts of the required pH control agent (5-15 grams) within the structure of a typical mercury vapor lamp.
U.S. Pat. No. 5,754,002, which was granted to Haitot et al. on May 19, 1998, relates to substantially preventing the formation of leachable mercury during disposal or TCLP testing of mercury vapor discharge lamps by incorporating an antioxidant in the lamp structure or in a test solution. A mercury discharge lamp is described which includes an effective amount, 0.05 to 10 grams per lamp, of an antioxidant such as ascorbic acid, sodium ascorbate, ferrous sulfate, ferrous gluconate and others. The stated purpose is to prevent the formation of soluble mercury compounds from elemental mercury in a landfill or in the TCLP test. The antioxidant is incorporated into the base end cap cavity, either in a base cement, or in an inert water soluble binder. The method described in the U.S. Pat. No. 5,754,002 patent adds cost and a component that has no other function than to pass the TCLP test. Furthermore, the stated purpose of preventing formation of water soluble mercury from elemental mercury does not address the water soluble mercury compounds formed during lamp operation. In this regard, the described ferrous salt is relatively ineffective in reducing soluble mercury concentration to less than 0.2 parts per million.
Some commercially available fluorescent lamps meet the requirements of the TCLP. Among these are, for example, linear four-foot lamps that utilize iron shields around the electrodes and contain three to five milligrams of mercury. The combination of the iron (low carbon steel) shields and soluble mercury content of less than five milligrams enables the lamps to meet the requirements. However, such an approach is unsuitable with significantly larger amounts of up to ten milligrams or more of soluble mercury. It is known that elemental mercury in lamps is converted to soluble mercury compounds during the lamps""s operating life. For at least a fraction of typical commercial lamps, the conversion significantly exceeds three to five milligrams of mercury. The failure of lamps before rated lifetime is unacceptable, and therefore a mercury dose significantly greater than three to five milligrams is desirable. An additional disadvantage of lamps with iron cathode shields is that the incorporation of the shield and the capsule for dosing mercury adds significant complexity and expense to the manufacture of the lamp.
All of the foregoing attempts to reduce the leaching of soluble mercury during the TCLP test and disposal involve the addition of significant complexity to the manufacture of fluorescent lamps and therefor added cost. In addition, many of such attempts do not employ standard lamp components. To the contrary, components are added solely to enable passing of the TCLP test. Further, some attempts require a low practical limit on the mercury content of the lamp, and such lamps may not pass the TCLP test unless the mercury content is very low or the component added to solve the problem is too massive to be practical.
In addition to all of the foregoing, conventional fluorescent lamp bases are typically fabricated from aluminum or brass. Such bases do not satisfactorily reduce leaching of soluble mercury. In addition, aluminum bases have a tendency to deform during transportation and during the base and lamp manufacturing process. Deformed bases result in manufacturing losses and therefore added costs.
It is an object of the present invention to provide an improved lamp base.
It is another object of the present invention to provide a lamp base which aids in the conversion of soluble mercury to elemental mercury.
A further object of the present invention is to provide a lamp base having improved resistance to deformation during transportation and during base and lamp manufacturing.
Another object of the present invention is to provide an improved lamp having the lamp base(s) of the present invention.
Yet another object of the present invention is to provide a lamp having a lamp base which aids in the conversion of soluble mercury to elemental mercury during disposal of the lamp.
A further object of the present invention is to provide a lamp having a lamp base which aids in the conversion of soluble mercury to elemental mercury and is less costly to fabricate than lamps heretofore provided.
Another object of the present invention is to provide a lamp which aids in the conversion of soluble mercury to elemental mercury yet requires only a standard lamp component.
It is a further object of the present invention is to provide an improved lamp which permits the use of practical amounts of mercury and passes the TCLP using only a standard lamp component.
Yet another object of the present invention is to provide a low cost fluorescent lamp base that effects the reduction of soluble mercury to elemental mercury in tests, such as the TCLP, and in lamp disposal operations.
A further object of the present invention is to provide an improved lamp base which can reduce at least 10 mg of soluble mercury in otherwise conventional four foot T8 and T12 lamps to less than 0.2 milligrams per liter in the TCLP.
This invention achieves these and other objects by providing a lamp base which is fabricated from a material which substantially prevents formation of leachable mercury in lamp disposal and testing procedures. A mercury vapor discharge lamp is also provided. Such lamp comprises an envelope of light transmitting vitreous material. The envelope contains an inert starting gas and a quantity of elemental mercury, and electrodes sealed in the envelope for establishing an arc discharge. The electrodes are electrically connected to respective lamp connectors. At least one lamp base is provided which is fabricated from a material to substantially prevent formation of leachable mercury in lamp disposal and testing procedures. In a preferred embodiment the lamp base of the present invention is fabricated from steel, and in particular, low carbon steel.